The conventional vehicle speedometer is a relatively simple device used to monitor the speed of a vehicle. How fast a vehicle actually moves at any instant is relatively unimportant but the speedometer itself has real importance in that by offering live speed data, it can be used by a vehicle's driver to estimate other more substantive external effects, such as his travel time, his fuel consumption, his miles traveled, his required distance to brake, his speed relative to other cars and to prudence and legal limits, and also, particular to this invention, the vehicle's driven speed relative to the vehicle's engine's rotation speed.
The internal combustion engine is often a powerful device, but it is a finicky one, capable of operation at a relatively narrow range of speeds. To adapt a typical internal combustion engine (or any engine or motor) to the task of motivating a heavy automobile rapidly between speeds as low as zero and as high 100 mph or more, it requires a scaling device typically embodied by a multiple gear ratio, fixed-ratio transmission. A transmission, in serving the task of rapidly scaling an engine's speed and torque against a vehicle's momentum, is subject to violent forces in the hands of the careless driver, particularly where a driver is attempting a quick gearchange with a manual transmission. Over time, a driver learns where appropriate engine and vehicle speeds coincide for each ratio of the transmission, but usually this results in either abuse of the vehicle's clutch or driving so conservatively, relative to a vehicle's performance envelope, as to render much of the reason for buying a vehicle of a given set of performance abilities redundant and unused.
To assist consumers in monitoring engine speed relative to a vehicle's speed, vehicle manufacturers currently provide most vehicles with a second speed indicator, a tachometer, specifically for the vehicle's engine. Alongside a vehicle's speedometer, a driver can thus see the actual rate of engine speed relative to the vehicle's speed as it accumulates, and better estimate when to shift between gears. However, this creates the problem of there being too many things moving around in front of the driver. To truly pay attention, a driver has to constantly flick his eyes back and forth between the road, the speedometer, and the engine speed tachometer, and shift gear accordingly. With a single gauge, less time is spent with one's eyes off of the road.
Also, since both the vehicle's wheels and the vehicle's engine are rotating elements, it has usually been easier and cheaper to design reliable and accurate gauges which merely rotate the tip of a needle around an arc. Therefore, to track vehicle speed and engine speed, a driver has to locate the tip of a needle somewhere within that arc, rather than looking for a single point within the vehicle that indicates both vehicle speed and engine while additionally moving in a purely translative fashion to convey rate of acceleration. Separate dials, despite the additional information about the specific speed of the engine, still require the driver to guess about how vehicle speed is specifically related to the engine speed in each gear to facilitate a smooth gearchange, especially for downshifts while braking for a turn in predication of rapid acceleration on exiting the turn. It is therefore desirable to provide a single gauge that allows tracking of both vehicle speed and engine speed to facilitate quick scans of the instrument panel as well as to provide a visual relation between vehicle speed and engine speed, in order to guide adjusting the two variables, to clearly change gears.
Of late, however, the providing of separate tachometers and speedometers horizontally and prominently displayed alongside one another has become so common among manufactured automobiles that it is desirable to provide a single device that can be adapted to a variety of tachometers and speedometers already in production so that many consumers can attach a combined single-point and engine speed tachometer to their existing car without having to wait for a major car manufacturer to offer it as an option.
One example of a combined, single point speedometer and engine speed tachometer is disclosed in U.S. Pat. No. 1,439,137 to White, which discloses a speed indicator having a single point of intersection for two needles, each needle displaying a corresponding value for the speed of a rotating device, with paths drawn on the background of the gauge to illustrate when the relative rotation speeds of the two devices correspond to gearing ratios between the two devices. The device uses two typical pure rotation tachometer devices and uses cord and pulley means from the rotating devices to translate the rotation movement into horizontal and vertical displacement movement.
However, the Relative Speed Indicator disclosed in the '137 patent is not particularly suited for retrofit into the typical mass-produced automobile, especially those that are fitted with traditional side-by-side pure-rotation tachometers for vehicle speed and engine speed. The arrangement of the '137 patent only facilitates the desired graphical display when the tachometer devices of the automobile can be arranged perpendicularly to the line-of-sight of the driver, which is completely at odds with the current practice of the mass-manufactured automobile. Also, in the two embodiments of the '137 patent, atypical calibration of the axes and gradations (not evenly spaced) will be required for the device to accurately display the correct rotation speeds and gearing ratio relationships for the device to convey accurate information to the driver of the vehicle about the two rotating devices, particularly in the preferred embodiment in which one of the needles is pinned at one end to the zero-point and is pulled about its free end by the engine speed tachometer. In such an arrangement, while the free end of that needle is pulled by the engine speed tachometer at a rate proportional to the change in the engine speed, the speed of the point at which the needle intersects with the bottom of the graduated window will only move at a rate that becomes more and more erroneous with the accumulation of engine speed, since the point at which the needle crosses that bottom-of-the-window axis travels further and further down the needle. To compensate, the graduations could be arranged out-of-proportion as the engine speed increases, but this is not claimed in the '137 patent, since such a relationship would result in engine speed-to-vehicle speed ratios that could not be illustrated on the background as simple horizontal lines. They would have to progressive slope with increasing gear ratio and more and more severely as they traverse the background of the gauge, as opposed to the claimed parallel lines of the '137 patent. The alternate embodiment is shown in Figure '137's FIG. 7, in which it is presumed that the relative speeds are arranged on perpendicular rather than parallel axes, and both needles traverse the background of the gauge in purely translative manner, as opposed to one needle rotating about the zero point. In such an arrangement, the gearing ratios between the two devices are correctly illustrated as lines that converse about the zero point. Such a device is more logical to read, as it assigns each speed a single dimension, either horizontal or vertical, rather than asking the driver to remember which of the horizontal axes is correlated with which rotating device. However, given the nature of how such a device could feasibly be produced, the device's lines would have to be set during the production process and would only correlate to a single instance of a vehicle's gearing. The present invention is designed with fit to a multitude of vehicles in mind. The invention of the '137 patent could be rendered useless by the driver's merely changing a vehicle's gearing set-up by installing a different transmission or differential, whereas the device of the present invention would only need minor adjustment.